Ductile iron is also referred to as nodular or spheroidal graphite cast iron and ductile iron castings are highly advantageous as compared to gray iron castings because of their ductility and because they have increased tensile strength and resistance to impact as compared to ordinary gray iron castings. Also, ductile iron has much better castability characteristics than steel and for an increasing number of uses, ductile iron castings can be used where steel castings or forgings were formerly required.
Ductile iron is produced by innoculating molten iron of the proper composition with a nodulizing agent that causes graphite which is ordinarily present in gray cast iron in flake-like form to assume a nodular or spheroidal form. It is this change in the characteristic of the graphite present in the cast iron that appears to give the ductile iron its advantageous characteristics. Whereas numerous nodulizers are known, magnesium is usually preferred because of its effectiveness, availability and relatively low cost.
Magnesium and the other nodulizing agents present problems because they have boiling points below the pouring temperature of cast iron and because they are easily oxidized. Because of these characteristics, attempts to add nodulizing agents in the conventional manners employed in adding alloying agents to molten iron present serious difficulties in that the addition of the agents to a ladle, for example, is likely to result in a violent pyrotechnic reaction that presents a hazard to foundry personnel and contamination of the foundry atmosphere. Also, some of the nodulizing agent is oxidized or otherwise lost, making it necessary to supply nodulizing agents in amounts in excess of the amounts required in order to compensate for "fading" or a reduction in the amount of nodulizing agent in solution.
In my Canadian Pat. No. 1,076,319, I disclose an apparatus and method particularly adapted for the automatic production of nodular iron castings. The patented apparatus is capable of rapid and economical production of high quality nodular iron castings and advantageously minimizes pyrotechnics developed during mold pouring.
The apparatus disclosed in my Canadian patent includes a mold having a casting cavity and a reaction chamber in which the nodulizing agent is deposited. The reaction chamber has an open top which is at the level of the adjacent upper or top surface of the mold and the floor of the chamber is at a level above the uppermost portion of the casting cavity. The chamber is connected at a level that is above the uppermost portion of the casting cavity. A cover or sealing member comprised of refractory material makes sealing engagement with the upper surface of the mold surrounding the open top of the chamber.
The sealing member is supported by a launder that is movable between a lower position with the sealing member in engagement with the flat, upper surface of the mold and a raised position in which the sealing member is disengaged from the upper surface of the mold; these movements of the sealing member are in directions substantially normal to the upper surface of the mold so that no appreciable sliding movement of the sealing member takes place with respect to the mold. The sealing member is raised from the mold after the predetermined amount of molten metal has been completely drained from the launder and the conduit into the mold and the mold is then free to be advanced past the sealing member for cooling and removal of the solidified casting.
As described in my Canadian Pat. No. 1,076,319, the apparatus is most advantageously used in combination with an automatic molding machine which may be any conventional type, such as the Disamatic molding machine produced by Dansk Industri Syndikat A/S. This machine produces individual molds successively and deposits them on the pouring rail of the machine. With this type of machine, each mold abuts a preceding mold and, in turn, is abutted by another following mold. A hydraulic ram associated with the molding machine pushes the whole series of molds along the pouring rail simultaneously. Thus, each mold is indexed a predetermined distance remaining stationary while another mold is deposited on the pouring rail, after which the entire series is again indexed. It is during the intervals when the molds are stationary that the nodulizing agent is introduced into the molds and the molds are poured.
As the molds travel along the pouring rail, they pass beneath an alloy-dispensing mechanism which may be of a known type and is arranged to deposit a predetermined amount of nodulizing agent in each mold. Each mold stops in registry with the alloy dispensing mechanism and while it is stopped, a predetermined amount of alloy is deposited into the nodulizing chamber of the mold. Thereafter, the mold is advanced beyond the alloy-dispensing mechanism and another mold is advanced into a position in registration with the alloy-dispensing mechanism where it, in turn, will receive a discrete amount of nodulizing material.
The molds continue to be advanced step-by-step down the pouring rail, passing a pressurized furnace which is arranged to deposit a predetermined amount of molten iron in a predetermined time into each mold as it stops on the pouring rail opposite the furnace. The batch quantity of molten metal is discharged from the furnace into a launder, and from the launder, the metal flows by gravity into the mold and to the casting cavities thereof as described in detail in my aforementioned Canadian patent. Each mold stops long enough opposite the furnace to permit the discharge of the required amount of iron into it and after this has taken place, the molds are all indexed another step and a following mold is disposed in pouring position opposite the furnace.
After the molds are poured, they continue on their step-by-step movement down the pouring rail where the castings are solidified, then into a cooling conveyor of known construction. Ultimately, the castings are removed from the molds, trimmed, inspected and tested. Conventional exhaust hoods may be placed over the conveyor as desired. Molten iron may be supplied to the furnace by conventional melting furnaces disposed on the other side of the furnace from the pouring rail.
The aforedescribed prior art apparatus and method works extremely well but there may be occasions when it is desired to entrap impurities before they flow into casting cavities by way of runners or downsprues. It may also be desired to allow a greater period of time for reacting the nodulizing agent with the molten metal to increase the homogeniety of the alloyed mixture.
Inasmuch as any molten metal remaining in the mold cavities forming the reaction chamber and the runners after the pour is completed becomes scrap, which must be trimmed from the casting, the volumes of the cavities formed in the mold by the reaction chamber and the runners should be minimized to maximize yield efficiencies. In addition, the shallower the depth of the open reaction chamber, the more readily visible is the nodulizing agent to a pourer responsible for ensuring the presence of the agent in the reaction chamber before making the pour.